The present invention relates to lead-tin alloy solders, in particular to dispersion strengthened lead-tin alloy solders. The increased demand for miniaturization and the use of surface-mounted technology in the microelectronics industry place more stringent requirements on solders which serve as mechanical interconnects as well as electrical contacts between surface mounted devices (SMD) and printed circuit substrates.
Solder joints employed under these conditions frequently undergo degradation due to thermal cycling. Such thermal cycling may occur during high temperatures employed in circuit board processing, and, as a result of temperature fluctuations in the ambient and component power cycling Thermal cycling tends to degrade solder joints because of the development of shear stresses arising from inherent thermal expansion mismatch normally occuring between most components and substrates. Service temperatures at which these assemblies are employed frequently are high relative to the absolute melting point. As a result, the microstructures of these solders tend to be highly unstable under typical operating conditions. Such unstable microstructures lead to recrystallization and grain growth resulting in a loss of strength. Because of this, soldered joints are likely to be prone to fatigue cracking.
In order to eliminate or reduce the extent of these problems, it has been found very useful to stabilize the microstructure of solders after reflow. In order to achieve such stabilization, two procedures may be employed. One approach is precipitation hardening in which a fine and homogeneous dispersion of an equilibrium phase precipitates from the solid solution. In the other approach, dispersion strengthening is employed in which additional material is introduced into the solder by external means.
While both approaches may lead to solder joints of higher strength and better control of the microstructure, there are certain problems in the applications of either of these methods. The method of precipitation hardening is difficult to employ for several reasons. Among these reasons are the materials which are to be added must be compatible with the metal matrix; precipitation must occur only in the solid state over a highly restricted temperature range; the resultant precipitates are quite likely to coarsen rapidly as a result of the same diffusional processes that allow their formation, and these coarsened precipitates may be detrimental to the mechanical properties of the soldered joint.
The other method that may be employed, the use of extrinsic dispersoids has advantages over the use of the precipitation hardening method in that better control of the solder microstructure is possible, and there is a greater choice of strengthening materials that may be incorporated into the metal matrix.
However, solders that are used in the microelectronic industry generally must undergo melting and resolidification before being employed to form a soldered joint. During such procedures, solid phases are normally rejected from the melt or particle agglomeration occurs in the melt. Therefore, it is considered to be necessarily implied that for the production of useful dispersion strengthened lead-tin alloy solders that the material dispersed in the solder, resist rejection from the melt and resist coarsening when present in the solder joint.
Hill et al, U.S. Pat. No. 4,018,599, shows dispersion strengthening of gold contacts by dispersing CeO.sub.2 in the gold. Hill et al, U.S. Pat. No. 4,018,630, shows dispersion strengthening of silver contacts by dispersing CeO.sub.2 in the silver. Bornstein, U.S. Pat. No. 4,834,939 shows a silver base contact material in which cadmium oxide and nickel particles are dispersed. However, none of these patents are concerned with dispersion strengthening of lead-tin alloy solder.
Swiss Patent CM 317,776 shows a solder alloy of copper nickel in which 1.1 to 1% by weight zinc is dispersed. There is no suggestion in the Swiss patent that the zinc results in dispersion strengthening of the solder. In addition, there is no suggestion in the Swiss patent that the copper-nickel alloy of the Swiss patent which is primarily used for biasing is at all relevant to a lead-tin solder employed in making solder joints.
JP 54-152493 shows that solder joints formed by gold-tin alloy may be improved by the incorporation into the alloy of a small amount of nickel. The Japanese patent also teaches that improvement in the strength and hardness of the soldered joint results and that the nickel forms Ni.sub.3 Sn.sub.4 during formation of this soldered joint by heating.
Strengthening of the joint that is produced by the method of this patent is due to precipitation hardening. As a result, the distribution and amount of the precipitation that is formed (Ni.sub.3 Sn.sub.4) and consequently the strength of the joint depends upon the joining procedure and the subsequent treatment employed.